This invention relates to improvements in the control of multi-channel paper machine distributors for maintaining uniform stock ratio in multi-ply paper webs.
Substantial improvement in the laminar structure, speed of operation, simplification of apparatus and savings in power consumption have resulted from developments in multi-channel head boxes or distributors. Examples of such multi-channel distributors are disclosed in U.S. Pat. Nos. 3,598,696; 3,839,143 and 3,923,593, all of which are owned by the same assignee as the present application. Characteristic of such distributors is the confluence at the slice opening of the distributor of the fibrous paper stock having the desired respective characteristics from the several, generally three, channels within the distributor which converge toward the slice opening. From the slice opening the multiple stock layers pass to the former for dewatering on or between a fabric or wire belt system and from which the composite sheet passes on through the finishing rolls of the paper making machine. These three-channel distributors are especially useful in producing paper of the kind commonly referred to as linerboard comprising a relatively thick and low bursting strength largely wastepaper heavy stock inner layer between two outer layers of generally virgin pulp heavy stock applied in relatively thin layers providing desirable finish and burst strength in the finished sheet. Before start of operation the slice opening is set to establish a basic water rate according to experience or bases on a prediction for the grade and speed of paper to be run. In addition the total head setpoint, the outer liner ratio setpoints, and the three basic weight setpoints are adjusted to the desired values. The forming section of the machine is then turned on and when it has reached running speed, the "start" button for the pumps for each of the stock supplies for the respective distributor channels are actuated. Then when the sheet is established on the machine, the basic weight is checked, the basic weight of any layer is independently readjusted as required and if formation or other sheet properties require, a change in spouting velocity or water rate or consistency. Heretofore this has required individually adjusting the flow rate or consistency for each of the channels of the distributor. Close supervision has been required to avoid undesirable ratio variances in the outer layers relative to the inner layer of the web as produced with the aid of the plural channel distributor. Another problem has been to maintain the total head at the slice opening properly controlled for the desired spouting velocity properly corelated with the speed of the former of the paper making machine.